The invention relates to a system for receiving and decoding digital signals in accordance with two image definition levels and including two channel decoders followed by a source decoder. This invention is applicable, for example when the same television program is broadcast in accordance with two definition levels (normal definition and high definition), and particularly when the broadcast is in conformity with the MPEG standard (whose essential characteristics will be described hereinafter).
For transmitting television signals in accordance with two definition levels, which will hereinafter be referred to as TV and HDTV transmissions, respectively, a choice is often made to ensure that TV definition signals are better protected against transmission errors than HDTV definition signals. This characteristic feature is particularly important in that it provides an easier reception of TV signals by means of a smaller antenna, a less expensive receiver, etc. However, in certain conditions such as a distant position of the transmitter or various perturbations of the transmission in general, the HDTV signals may not be fully usable for decoding or may even be completely unusable because they are less well protected than the TV signals.